Structures, such as a bridges, buildings, heavy equipment, aircraft, and guns are subject to stresses from energetic events and damaging events as well as from ordinary use. An energetic event may be the firing of a gun, such as a handgun, a rifle, an aircraft gun, artillery, or a rocket launcher. A damaging event may be a collision, an explosion, an earthquake, or a fire. A damaging event may also be caused when a structure or vehicle is hit by a bullet, missile, or shrapnel. A damaging event can also be caused by excessive loading during otherwise ordinary use. Damage can accumulate over time from repeated use, particularly repeated use with excessive loading. Damage can also result over time from corrosion, thermal cycling, or humidity during otherwise ordinary use. Damage can also occur from degradation produced by an excessive number of ordinary uses.
Schemes to test structures for damage have been proposed, as described in the '731 application. But no completely passive scheme has been in place on structures to quickly sense the event that caused the damage or to electrically record the damaging event almost immediately after it occurs.
Sensors, signal conditioners, processors, and digital wireless radio frequency (RF) links continue to become smaller, consume less power, and include higher levels of integration. The combination of these elements can provide sensing, acquisition, storage, and reporting functions in very small packages. Such sensing devices have been linked in wireless networks as described in the '127, patent and in the '9224, '194, '481, '541, '731, '637, '066, and '436 applications.
Networks of intelligent sensors have been described in a paper, “Intelligent Sensor Nodes Enable a New Generation of Machinery Diagnostics and Prognostics, New Frontiers in Integrated Diagnostics and Prognostics,” by F. M. Discenzo, K. A. Loparo, D. Chung, A. Twarowsk, 55th Meeting of the Society for Machinery Failure Prevention Technology, April, 2001, Virginia Beach.
Wireless sensors have the advantage of eliminating wiring installation expense and weight as well as connector reliability problems. However, wireless sensors still require power in order to operate. In some cases, sensors may be hardwired to a vehicle's power system. The wiring required for power defeats the advantages of wireless sensors and may be unacceptable for many applications. In addition, if a power outage occurs, critical data may be lost, at least during the time of the power outage.
To counteract anticipating degradation with each firing, military aircraft guns are ordinarily scheduled for tear-down and inspection every 15,000 rounds or every 18 months, whichever occurs first. Schemes have been proposed to count the number of rounds fired by a particular gun while in ordinary use, such as described in U.S. Patent Application US2003/0061753 to Glock filed Sep. 23, 2003, and US2004/0200109 to Vasquez, filed Feb. 6, 2004. However, these schemes have required the use of batteries, which themselves require maintenance, to provide power for their detecting, data storage, and communication electronics.
Similarly, most prior wireless structural monitoring systems have relied on continuous power supplied by batteries. For example, a paper “An Advanced Strain Level Counter for Monitoring Aircraft Fatigue”, by Weiss, Instrument Society of America, ASI 72212, 1972, pages 105-108, 1972, described a battery powered inductive strain measurement system which measured and counted strain levels for aircraft fatigue. The disadvantage of traditional batteries, however, is that they become depleted and must be periodically replaced or recharged. This additional maintenance task adds cost and limits use to accessible locations.
Given the limitations of battery power, there has been a need for systems which can operate effectively using alternative power sources. Energy harvesting from vibrating machinery and rotating structures to provide power for such sensing devices and for wireless networks of sensors and/or actuators has been described in the commonly assigned '693 patent and in the '976, '679, '632, '642, and '731 applications.
A paper, “Energy Scavenging for Wireless Sensor Networks with Special Focus on Vibrations,” by S. Roundy et al., Kluwer Academic Press, 2004, and a paper “Energy Scavenging for Mobile and Wireless Electronics,” Pervasive Computing, by J. A. Paradiso & T. Starner, IEEE CS and IEEE ComSoc, Vol 1536-1268, pp 18-26, 2005, describe various strategies for harvesting or scavenging energy from the environment.
U.S. Pat. No. 6,407,483 to Nunuparov, filed with the PCT on Oct. 29, 1998 and in the U.S. on Apr. 27, 2000, U.S. Patent Application US 2005/0087019, to Face, filed Oct. 25, 2004, the '693 patent, the '642 application, and the '777 application describe systems that harvest ambient energy for providing electrical power. These systems can provide power autonomously because they do not require traditional battery maintenance.
However, these energy harvesting systems have not been optimized for use on structures, such as aircraft, containers, and weapons, for use in networks, or for use in monitoring structures and equipment that may be subject to specific events, such as a damaging event, or the normal operation of an apparatus, such as the firing of a gun or the opening of a door. Thus, an improved system for monitoring is needed that can effectively use energy of an event for recording information about the event, and this solution is provided by this patent application.